Iododeoxyuridine (IUdR) is a known radiosensitizing drug which is being delivered as a constant intravenous infusion for 12 hours or 24 hours/QD for up to 14 days in patients with high grade primary brain tumors and other poorly radioresponsive tumors. The drugis being used as a clinical radiosensitizer and being combined with high-dose radiation therapy in an attempt to improve the response rate of these poorly radioresponsive tumors as well as to assess the toxicity both local and systemic of this radiosensitizer. Twenty-four patients have been entered onto the trial using a 12 hour infusion and 28 patients entered on the trial using 24 hour infusions. Patients are treated with twice daily fractions of radiation therapy given in two separate sessions and combined with two separate infusions of IUdR.